This invention concerns systems and apparatus for removal of biological tissue samples and, in particular, systems and apparatus for refractive vision correction.
Various techniques and devices are known in the art for the removal or slicing of thin layers of biological tissue. These instruments are generally referred to as surgical microtomes. When the instruments are specifically designed for the removal of corneal tissue they are often referred to as keratomes or micro-keratomes.
It is often desirable in the course of human therapy to remove a thin layer of biological tissue intact. This is especially true in the course of biopsying tissue specimens for the presence of cancerous or otherwise abnormal cells. For example, in the course of cancer diagnosis and/or treatment, biopsy samples are routinely taken and analyzed for the presence of cancerous cells. The thin tissue biopsies are examined microscopically as a surgical procedure is carried out to ensure that the margins of tumor excision are properly delineated and excessive excisions of healthy tissue are avoided.
Biopsies are also taken for diagnostic purposes in the course of other surgical procedures or in the course of health care, generally. For example, when a gynecological xe2x80x9cPAP smearxe2x80x9d test turns up a positive result for abnormal cells, cervical biopsy samples are necessary to confirm or negate the presence of cancer cells in the cervix. Biopsies are also taken in the course of various laproscopic examinations for similar purposes in order to confirm or negate the presence of abnormal or cancerous cell conditions.
The examination of biopsy samples is often a tedious process. The ability to take a standardized biopsy sample of a defined volume or thickness of tissue is highly desirous in order to facilitate either manual and automated examination of the biopsy sample. Many conventional microtomes cannot provide this standardization of biopsy samples.
Thus, there exists a need for better surgical resection instruments, generally, which can remove thin sections of biological tissue for examination purposes.
Additionally, in the field of surgical correction of refractive vision disorders, keratomes are used for various purposes. These purposes include the removal of abnormal growths in the cornea, preparation of damaged eyes for corneal transplants, preparation of eyes for other surgical procedures and direct surgical corrections of refractive disorders.
Considerable interest has been recently generated in a variety of techniques for reshaping the cornea for refractive vision correction. These techniques are based on the observation that most of an eye""s refractive power is contributed by the corneal curvature itself (with the remaining refractive power being provided by the lens of the eye located inside the ocular globe). For people suffering from near-sightedness (myopia), it has been recognized that a slight flattening of the corneal curvature can correct this condition if properly applied. Conversely, correction of far-sightedness (hyperopia) requires a steepening of the corneal curvature. Correction of astigmatism typically requires more complex reprofiling.
It has been suggested on a number of occasions that it is possible to correct refractive errors by mechanical sculpting of the cornea into an ideal shape and curvature. However, until very recently, there have been no tools suitable for this purpose. The anterior surface of the cornea is covered with a thin layer of epithelial tissue followed by a membrane-like structure known as Bowman""s layer. Typically, Bowman""s layer is about 30 micrometers thick, although it may vary from as little as 10 micrometers to over 50 micrometers in thickness.
Below Bowman""s layer lies the stroma proper of the cornea. This stromal tissue is approximately 450 micrometers in thickness, although it also varies from individual to individual. Stromal tissue is composed of a highly organized matrix of acellular collagen. The Bowman""s membrane which lies above it is less regular and denser.
Efforts at mechanical sculpting of the cornea have been largely unsuccessful to date because even the sharpest metal (or even diamond) blades are incapable of producing precise ablations of corneal tissue with the necessary accuracy. The irregularity of Bowman""s layer is a further complicating factor which has stymied mechanical attempts at wide-area sculpting of the anterior surface of the cornea.
Attempts have been made to achieve corneal reprofiling in other ways. For example, in a procedure known as radial keratotomy (RK) the cornea is incised with radial cuts which cause the overall structure of the cornea to relax and flatten. While moderate success has been achieved with RK procedures, the result is far from ideal. In such procedures, the anterior surface of the eye contains ruts which resemble the spokes of a wheel after the incisions have healed and the actual corneal curvature is far from the ideal spherical shape that is desired. Nonetheless, millions of people suffering from refractive disorders have undergone RK procedures in the hopes of permanently correcting their vision.
In another approach, designed to avoid the complications of Bowman""s layer, a thicker anterior segment of the cornea (typically including Bowman""s layer and several hundred micrometers of stromal tissue) is removed, frozen and lathed on its posterior (inside) surface. This reshaped corneal cap or xe2x80x9clenticulexe2x80x9d is then thawed and replanted onto the cornea. In this procedure, often referred to as keratomileusis, the integrity of the Bowman""s layer is maintained but a much more invasive procedure is required to remove the corneal lamella and then reshape it in a frozen state.
In another alternative surgical procedure, a anterior segment of the cornea is again removed (or partially severed and displaced) so that the stromal bed can be mechanically reshaped (e.g. with a scalpel-like instrument). Because Bowman""s layer is removed or displaced intact in such procedures, mechanical instruments (microkeratomes and the like) have had moderate success in resculpting the stroma proper. After the stromal bed has been surgically reshaped, the anterior lenticule is replaced. Again, this procedure has the advantage of avoiding mechanical shaving Bowman""s layer, albeit at the expense of a deeper penetration into the stroma.
Recently, a new class of tools has become available to ophthalmologists to perform corneal surgery. This class of tools employs high energy pulses of ultraviolet radiation, typically from excimer lasers, to ablate thin layers of corneal tissue by a process known as xe2x80x9cphotodecomposition.xe2x80x9d This laser vision correction process relies upon the ability of such laser radiation to remove extremely thin layers of corneal tissue within an exposed area without thermal damage to adjacent tissue. In one type of procedure known as photorefractive keratectomy (PRK), the laser beam is either repeatedly scanned across the cornea or otherwise controlled to expose the cornea to a beam of different shape or size over time so as to effect a cumulative reprofiling of the corneal surface. In many PRK procedures, ablation is largely confined to Bowman""s membrane and the laser radiation achieves very smooth and reproducible results. For patients with high dioptric errors, the ablation will also penetrate into the stromal tissue, again with typically very good results. Nonetheless, the systems and apparatus necessary for achieving laser vision correction are extremely complicated to operate and maintain.
In a particular class of PRK procedures known as Laser Assisted In Situ Keratoplasty (LASIK), a keratome is still used to remove (or hingedly displace) an anterior lenticule of the cornea (in much the same way as in the procedures that involve mechanical sculpting of the stroma) so that the laser can be used to ablate only stromal tissue. Again, like mechanical sculpting procedures, the anterior lenticule is replaced following the procedure with Bowman""s membrane intact. This LASIK procedure is also very promising but likewise requires highly complex, difficult to maintain, equipment.
Conventional keratomes typically include a suction device that secures the instrument to the eye and a drive mechanism that pushes a blade through a channel within the device to sever an anterior segment of the cornea. Examples of such devices include U.S. Pat. No. 5,496,339 issued to Koepnick on Mar. 5, 1996 and European. Patent Application Pub. No. 0 771 553 by H. Schwind GmbH published May 7, 1997. The precision of these instruments is inherently limited by the clearance necessarily provided between the blade and the upper and lower surfaces of the channel. To permit the blade to pass smoothly through the instrument, the gap must be larger than the thickest portion of the blade. This constraint introduces a degree of variability that limited the instrument""s utility particularly in achieving specific refractive corrections.
There exists a need for alternative techniques in order to reprofile the cornea. Simple mechanical devices that could achieve the desired degree of accuracy, especially in shaping Bowman""s membrane tissue as well as stromal tissue, would satisfy a long-felt need in the ophthalmic community.
Moreover, there exists a need for better keratomes, generally, to facilitate both mechanical and laser vision correction procedures. A better, more accurate keratome, would allow ophthalmic surgeons to perform therapeutic keratectomies (removing small regions of corneal tissue which exhibit abnormal growths or ulcers), resections of anterior corneal segments (as a first step in keratomileusis, stromal sculpting procedures, LASIK procedures and the like) and a variety of other surgical operations on the cornea.
Methods and apparatus are disclosed for removal of biological tissue slices or layers, for example, in the form of lamellar sections of a defined shape and thickness, employing a reference member that engages a target tissue site and cooperates with a cutter to remove the tissue segment or lamella. The cutter preferably includes a flexible cutting element such as a wire or band element that is brought into physical contact with a guiding edge of the reference member and then draw along a path defined by the guide edge through the tissue in order to sever, at least partially, a tissue section.
In one particularly useful aspect of the invention, methods and apparatus for keratectomy are disclosed employing an ocular reference member that engages the upper central region of the cornea and cooperates with a cutter to remove a lamellar segment from the cornea. Such lamellar resections are useful in preparing the cornea for further surgery (by mechanical or laser surgical techniques), or in performing refractive keratectomy directly upon the eye, or in treating (e.g., smoothing) the corneal surface to correct abnormalities, (e.g., to remove ulcerated tissue or otherwise improve the optical properties of the cornea).
Various structures can be used as cutters in the present invention, including flexible cutting elements such as wires, fibers, bands and the like. A driver is preferably employed to bring the cutter into physical contact with a the guiding edge of the reference member and then draw the cutter along the guiding edge and through the tissue. Exemplary driver mechanisms including motors which activate a sweeping motion, linear motive actuators, translating stages, pivoting actuators and rotating actuators.
The cutter preferably also moves in a linear direction (e.g. in a direction orthogonal to the path along which the cutter is drawn through the tissue) as it slices a lamella. This linear motion can be unidirectional or oscillatory.
In another embodiment of the invention the cutter can be a relatively rigid element (e.g. a very thin, sharply beveled blade) that likewise is drawn across a guiding edge of an ocular reference member to effect tissue resection. The blade can be connected to a wire, fiber, band or belt and its movement can be actuated by the same mechanisms as described above with flexible cutting elements. So long as the ocular reference member is incorporated into a flat support base that is flush with reference member (or subsumes the function of the guiding edge), the blade as it is drawn The guiding edge of the reference member defines a path through the tissue. The shape of the cavity and the geometry of the guiding edge define the shape of the sliced lamella. Because the cutter can be flexible or driven by a flexible driver assembly, the edging need not be planar and, in some applications, a non-planar guiding edge is desirable (to remove non-uniform tissue samples or, in the case or refractive surgery, to effect astigmatic corrections).
The invention can be used in refractive surgery either as a reprofiling device or, more simply, as a microkeratome to perform initial processing steps in order to prepare the cornea for mechanical sculpting or LASIK procedures. In refractive procedures, the reference member can have a distal end that defines a cavity of predetermined shape in order to capture a desired volume of tissue. When the cutter is drawn through the corneal tissue, the tissue within the cavity of the reference member is severed from the cornea. By proper choice of the cavity geometry, a new curvature will be imparted to the remaining corneal tissue. For example, a concave cavity will remove a volume of tissue that results in an overall flattening of the corneal curvature, thus correcting myopic vision errors. Other designs of the distal end can yield hyperopic and/or astigmatic corrections.
Moreover, the present invention can be used in mechanical sculpting procedures for dual purposes: first, to remove (or hingedly displace) an anterior portion of the cornea (with the epithelium and/or Bowman""s layer of the central optical zone unaltered) and then in a second step to reshape the overall curvature by removing a defined volume of corneal stroma tissue. After the lamella is removed from the stroma, the anterior xe2x80x9ccapxe2x80x9d can then be replaced over the sculpted region.
The guiding edge of the reference member need not define a planar path for the cutter. In fact, the guiding edge can be a non-planar shape in some applications, such as in reprofiling corneal tissue to correct astigmatisms. The ability to perform non-planar resections is an important advantage of the present invention. Resections of tissue with convention microtome and microkeratome device are limited to planar slices because of the inherent geometry of knife blades and the like as cutting elements. When a non-planar edge is incorporated into the reference member, it is also desirable in certain applications to also have an orientation marker for orienting the cutter and the reference member such that the path that the cutter follows through the tissue is defined and fixed. It is also desirable to include means for adjusting the orientation so that the user can select a particular azimuthal orientation.
In one embodiment of the invention, the reference member and cutter are joined by a pivot bearing and a driver causes the cutter element (e.g. a moving wire) to swing into engagement with the guiding edge of the reference member. The driver continues to draw the cutter through the tissue until the lamella is severed (or until a stop point is reached). The moving wire or band can be reeled in either a linear or oscillating fashion by one or more reel or spool mechanisms. The cutter element and the spools which facilitated the motion of the cutter can be designed as a disposable cartridge which is replaced with a new unit following each procedure.
The reference member can be hollow with a transparent distal end to serve as a viewing tube. The proximal end can include an eyepiece. The reference member can straight or folded (with internal mirrors) to permit viewing of the tissue section. Such viewing is particularly desirable in corneal resection procedures where the lamella to be removed or displaced is usually centered in the optical zone (or aligned with the pupil but offset nasally). The reference member can also include one or more means for securing the tissue segment including, for example, suction ports incorporated into the periphery or the cavity of the reference member, adhesive coatings, or mechanical means, such pins, teeth, or clamps. Alternatively a separate suction ring or other securement structure can be used in conjunction with the reference member.
The term xe2x80x9ccutterxe2x80x9d as used herein is intended to encompass any one of a a variety of cutting elements that can be drawn through biological tissue to effect a partial or complete resection of a tissue segment. It is often desirable that the cutters of the present invention are preferably flexible elements such that physical contact between the cutter and the guiding edge of the reference member ensures precise removal of a predictable, defined volume of tissue. However, in other applications a stiff blade-like structure can also be used advantageously. The term xe2x80x9cguiding edgexe2x80x9d is used herein to describe that portion of the reference member that interacts with the cutter to guide the cutter thorough the tissue. The term xe2x80x9ccavityxe2x80x9d is used herein to describe the shape of the distal end of the reference member such that the form of the cavity defines, at least in part, the volume of tissue removed. The term xe2x80x9cremovedxe2x80x9d is intended to encompass not only the complete severing of a tissue lamella but also the partial resection of such tissue, including procedures in which a lamella is partially severed and hingedly displaced for subsequent reattachment. The terms xe2x80x9clamellaxe2x80x9d and xe2x80x9clamellar segmentxe2x80x9d are used herein to describe the tissue segment removed by the action of the cutter; such lamella need not be flat (and in most instances it is not flat). The term xe2x80x9cadhesionxe2x80x9d is intended to encompass both frictionally adherent and adhesively bonded mechanisms for securing the reference member to the target tissue site.